Ultracentrifugation is routinely used to separate complex biological samples into their various fractions by taking advantage of small differences in density. The fractions or gradients of the stratified sample produced by ultracentrifugation are then analyzed by many known techniques such as the use of a fractionator. To analyze a stratified sample disposed in a centrifuge tube, the fractionator first uses a mechanism to puncture the bottom of the tube. Very dense fluid is introduced through the puncture which displaces the stratified sample upwards. The displaced fluid fractions of the sample are routed through an analysis module, such as a spectrophotometer, and collected as separate samples in test tubes held in a carousel.
There are two major weaknesses of the fractionator. First, it cannot be used with all centrifuge tubes, only those soft enough to be punctured. This limitation precludes the use of glass or polycarbonate tubes. Polycarbonate tubes are particularly important since they are used in very high speed centrifuges (i.e., about 100,000 rpm or more). Also, the fractionator destroys the stratified sample during analysis, thus precluding the possibility of repeated measurements on a single sample.